1. Field of the Invention
The present invention relates to a radio base station apparatus, a radio terminal apparatus, a radio communication system, a band assignment method and a program capable of providing a part of a period regarding a data transmission right which has been acquired by an initiator to a responder.
2. Description of the Related Art
The quality of service (QoS)-expanded wireless LAN standard IEEE 802.11e includes two types of access control methods: an enhanced distributed channel access method (EDCA); and an HCF controlled channel access (HCCA) method. When a wireless LAN terminal acquires a transmission opportunity (TXOP) period in which a wireless LAN terminal can transmit a plurality of items of data by using either of the two types of access control method, the wireless LAN terminal which has acquired the TXOP period can perform data transmission during the TXOP period in accordance with the IEEE 802.11e standard (refer to IEEE 802.13e Draft 13.0, IEEE P 802.11e/Draft 13.0, January 2005).
On the other hand, IEEE 802.11n intending to a high-rate transmission presents a bi-directional data transfer method capable of enhancing a transmission efficiency in a manner that the wireless LAN terminal which has acquired the TXOP period provides a part thereof to a responder for the data so as to perform a bi-directional communication by a piggyback method during the same TXOP period.
IEEE 802.11n differs from other existing IEEE 802.11 standard, prepares an Aggregation frame in which a plurality of items of data collected (i.e., aggregated) into one data frame then transmits the Aggregation frame as the one data frame so as to reduce overheads which have existed among each data frame (when not aggregated).
When acquiring the TXOP period for an Aggregation frame transmission in the EDCA method, an IAC-RAC frame exchange is performed, wherein an initiator transmits an initiator aggregation control (IAC) frame then the responder replies a responder aggregation control (RAC) frame after its short inter-face space (SIFS).
In the bi-directional data transfer, at the IAC-RAC frame exchange, the responder for the data writes the number of data frames which can be transmitted when the part of the TXOP period has been provided and a transmission data rate into the RAC frame to inform them. The initiator determines, from the value which has been described in the RAC frame, the part of the TXOP period to be distributed after transmitting the Aggregation frame. Hereinafter, this period is referred to as a reverse direction grant (RDG) Duration. The initiator writes the determined RDG Duration into the IAC frame to add it on the head of the Aggregation frame and transmits it after the SIFS from the reception of the Aggregation frame. The responder which has received the Aggregation frame has to notify a reception situation resulting from a Block Ack (acknowledgement) frame after the SIFS from the reception of the Aggregation frame from the initiator. In the use of the bi-directional data transfer method, when the Block Ack frame is replied after the SIFS, the data from the responder is transmitted together with the Block Ack by using the piggyback method which aggregates some data frames with the Block Ack frame to transmit them. At this time, the transmission time period of the Aggregation frame in which some data frames are aggregated in the Block Ack frame is not allowed to exceed the time period of the RDG Duration written in the IAC frame.
In this way, the initiator can distribute the part of the TXOP period acquired by the initiator to the responder.
When the responder transmits the Aggregation frame in the piggyback method, if the responder further requires the RDG Duration, it may further require the RDG Duration by insetting the number of data frames and the transmission data rate to the RAC frame and adding them to the head of the Aggregation frame to reply them (Refer to TGn Sync. Proposal Technical Specification, IEEE 802.11-04/889r1, November 2004).
However, the aforementioned conventional technique determines a value of the RDG Duration to be assigned to the responder by referring to the value written in the RAC frame notified just before, so that the conventional technique leaves a situation of a traffic stream (hereinafter referred to as a TS) which has been set between a radio terminal station and a radio base station and a situation of radio environment out of consideration. Accordingly, if required values from the responder are large and all required values are reflected to the RDG Duration, it is presented as a problem, such that the transmission period to satisfy the requirement of the TS of the initiator is employed to the data transmission from the responder and the requirement of the set TS cannot be satisfied.
Since the conventional technique transmits the Aggregation frame with the RDG Duration after the SIFS from the RAC frame reception, the conventional technique has to calculate the value of the RDG Duration in an extremely short time. Therefore, the conventional technique has to design a wireless LAN baseband LSI by means of hardware with a high operation speed. It is hard for the design by means of the hardware to perform a complicated calculation, and the design presents a variety of problems such that failures occurs frequently in its development and modifications are hard after the development.